-íssimo

Borrowed from Latin -issimus. The borrowing occurred during the Renaissance by influence of Italian^[1].

-íssimo (adjective-forming suffix, feminine -íssima, masculine plural -íssimos, feminine plural -íssimas)

1. forms the absolute superlative

   ‎caro (“expensive”) + ‎-íssimo → ‎caríssimo (“extremely expensive”)

   ‎feio (“ugly”) + ‎-íssimo → ‎feiíssimo (“extremely ugly”)

• Though both are called superlatives, Portuguese words suffixed with -íssimo are not equivalent to English words suffixed with -est or preceded by most. Instead, they are equivalent to using the adverbs very or extremely to qualify the adjective.
• The following ending(s) change(s) in words appended with this suffix:
  • ⟨-ão⟩ (as a non-verb suffix, incl. plurals) → ⟨-on⟩
  • ⟨-ã(o)⟩ (incl. plurals) → ⟨-(i)an⟩, ⟨-am⟩, or ⟨-(i)on⟩, depending on the base word's etymology
  • ⟨-m⟩ (incl. plurals) → ⟨-n⟩
  • ⟨-vel⟩ (unstressed and adjectival, incl. plurals) → ⟨-bil⟩
  • ⟨-z⟩ (adjectival, incl. plurals) → ⟨-c⟩
• Adjectives ending in the semivowel ⟨i⟩ followed by a vowel lose the semivowel in nonstandard usage:

  ‎feio (“ugly”) + ‎-íssimo → ‎feíssimo (nonstandard)

  ‎feio (“ugly”) + ‎-íssimo → ‎feiíssimo (standard)

• Some words may loan the superlative from Latin in addition to forming one with this suffix:

  paupérrimo (loaned from pauperrimus) and pobríssimo, superlatives of pobre

• Colloquially, the syllable ⟨ssi⟩ may be repeated a number of times for emphasis:

  ‎grande (“big”) + ‎-issíssimo → ‎grandissíssimo (“very very big”)